In the past, there have been many different specific types of rotational drives for disc recorders, including magnetic disc recorders, or disc files, of the type used for data processing applications, which are rather sophisticated devices which record very large amounts of digitally encoded data at very high recording densities. In such devices, the disc (or discs) are usually mounted on a shaft or spindle, and the coupling between the shaft and a motor which drives it is usually done with a flexible belt drive. Usually, an AC motor is used in such a belt drive, with various schemes employed for speed regulation, which is highly important.